Mobile radio telephones are known as radio telephone apparatus and are used in vehicles such as automobiles. In an automobile telephone a dial signal and a speech signal are input at a handset unit disposed near the seat of the automobile. These signals are transmitted from a transceiver unit, installed for example, in the trunk of the automobile, to a base station through which a communication link with a remote land line telephone or a remote radio telephone is established. In such a conventional automobile radio telephone apparatus, the handset unit near the automobile seat cannot be disconnected from the transceiver unit in the trunk. Thus, a user cannot use the apparatus outside the automobile.
The above-noted problems associated with prior art radio telephones are addressed by the radio telephone apparatus described and claimed in application Ser. No. 249,817 mentioned above. The radio telephone described therein can be used inside or outside an automobile. FIG. 1 is a perspective view of such a radio telephone apparatus. The radio telephone apparatus comprises a portable unit 101 with an antenna 102 and an adaptor unit 103. Portable unit 101 and adaptor unit 103 are connected to each other by a coaxial cable 104. Adaptor unit 103 is supplied with power from a car battery (not shown) through a power cable 106 and is connected to an antenna 105 which is disposed outside the automobile. In accordance with the construction of the radio telephone apparatus, both portable unit 101 and adaptor unit 103 are equipped with a radio transmit/receiver circuitry. Thus, a user may communicate with another person by using portable unit 101 also or by using portable unit 101 attached to adaptor unit 103.
FIG. 2 is a block diagram of the radio circuitry which Applicants have found may be used in the radio telephone apparatus shown in FIG. 1. In the event that only portable unit 101 is available to the user, the output of power amplifier (PA) 106 is connected via switch 111 to antenna 102 which is provided on portable unit 101. The control and speech signals generated by signal source 107 are thus amplified by PA 106 and then transmitted via antenna 102. The amplification factor of PA 106 is determined by automatic power control circuit (APC) 108. APC 108 determines the amplification factor in response to a level control signal received by level control signal receiver 112 from a base station (not shown) and a feedback signal provided from the output of PA 106. APC 108 provide a control signal which establishes the amplification factor of PA 106.
In the event that adaptor unit 103 is available to the user along with portable unit 101, the output of PA 106 is connected to the input of power amplifier (PA) 109 via switch 111. Thus, signals generated by signal source 107 are amplified by PA 106 and PA 109 and then transmitted via antenna 105. In this configuration, APC 108 controls PA 106 in response to the level control signal and the feedback signal. Also, an automatic power control circuit (APC) 110 in adaptor unit 103 controls PA 109 in response to a level control signal received by level control signal receiver 114 from the base station (not shown) and a feedback signal provided from an output of PA 109.
Though the radio telephone apparatus illustrated in FIG. 2 represents a substantial improvement over such apparatus known in the prior art, it remains deficient in a number of respects. One such deficiency is that it is difficult to control the power level of the final amplifier stage, i.e., the output level of PA 109. This is because the input level to PA 109, i.e., the output level of PA 106, fluctuates in response to the control signal provided from APC 108.
Still, to constitute a radio telephone capable of transmitting and receiving radio frequency signals, an arrangement for receiving the signals is to be added to the above discussed radio circuitry of FIG. 2. U.S. Pat. No. 4,636,741 to Mitzlaff discloses several multi-level power amplifying circuitry for a portable radio transceiver attachable to a vehicle adaptor. Especially, FIG. 6 of the Mitzlaff patent discloses a multi-level power amplifying circuitry wherein both of the portable unit and the adaptor have a power control circuitry. According to the arrangement disclosed in the FIG. 6, since the transceiver is connected with the adaptor by plug, the transceiver becomes unavailable for handheld use and, accordingly, an additional handset is needed.
It might be considered to use cables between the transceiver and the adaptor. According to the prior art arrangement, however, to connect the transceiver with the adaptor, two cables are required, i.e., one is for transmission signals and one is for reception signals. A coaxial cable is typically available for transmitting high frequency signals. Since the coaxial cable is thicker and harder than a normal conductive lead line, if the two coaxial cables are twisted into a cable line, the cable line becomes thicker and harder much more, thereby the operability of the transceiver attached to the adaptor turns out to be worse.